The present invention relates to a process for preparing acrylic acid from methanol and acetic acid. The present invention also relates to the preparation of conversion products from acrylic acid thus obtained.
α,β-unsaturated acids, particularly acrylic acid and methacrylic acid, and the ester derivatives thereof are useful organic compounds in the chemical industry. These acids and esters are known to readily polymerize or co-polymerize to form homopolymers or copolymers. Often the polymerized acids are useful in applications such as superabsorbents, dispersants, flocculants, and thickeners. The polymerized ester derivatives are used in coatings (including latex paints), textiles, adhesives, plastics, fibers, and synthetic resins.
Because acrylic acid and its esters have long been valued commercially, many methods of production have been developed. One exemplary acrylic acid ester production process utilizes: (1) the reaction of acetylene with water and carbon monoxide; and/or (2) the reaction of an alcohol and carbon monoxide, in the presence of an acid, e.g., hydrochloric acid, and nickel tetracarbonyl, to yield a crude product comprising the acrylate ester as well as hydrogen and nickel chloride. Another conventional process involves the reaction of ketene (often obtained by the pyrolysis of acetone or acetic acid) with formaldehyde, which yields a crude product comprising acrylic acid and either water (when acetic acid is used as a pyrolysis reactant) or methane (when acetone is used as a pyrolysis reactant). These processes have become obsolete for economic, environmental, or other reasons.
More recent acrylic acid production processes have relied on the gas phase oxidation of propylene, via acrolein, to form acrylic acid. The reaction can be carried out in single- or two-step processes but the latter is favored because of higher yields (see, for example, DE-A 103 36 386). The oxidation of propylene produces acrolein, acrylic acid, acetaldehyde and carbon oxides. Acrylic acid from the primary oxidation can be recovered while the acrolein is fed to a second step to yield the crude acrylic acid product, which comprises acrylic acid, water, small amounts of acetic acid, as well as impurities such as furfural, acrolein, and propionic acid. Purification of the crude product may be carried out by azeotropic distillation. Although this process may show some improvement over earlier processes, this process suffers from production and/or separation inefficiencies. In addition, this oxidation reaction is highly exothermic and, as such, creates an explosion risk. As a result, more expensive reactor design and metallurgy are required.
Propylene can be produced from mineral oil with comparatively low production costs. In view of the foreseeable shortage in the fossil resource of mineral oil, however, there may be a need for processes for preparing acrylic acid from other raw materials.
WO 2005/093010 proposes the use of the two-stage heterogeneously catalyzed partial gas phase oxidation of propylene to acrylic acid. The propylene may be obtained from methanol. The advantage of such a procedure is that methanol is obtainable both from base fossil raw materials such as coal, for example brown coal and hard coal as disclosed in WO 2010/072424, and/or natural gas, as disclosed in WO 2010/067945. Both of these sources have a much longer lifetime than mineral oil. A disadvantage of the procedure proposed in WO 2005/093010, however, is that the selectivity to propylene based on methanol converted is less than 70 mol %, which is unsatisfactory (in addition to propylene, for example, ethylene and butylene are also formed).
WO 2008/023040, for example, has disclosed the preparation of acrylic acid and the conversion products thereof starting from glycerol, a renewable raw material. A disadvantage of such a procedure, however, is that glycerol is only feasibly obtainable as a renewable raw material essentially as a coproduct of biodiesel production. And the current energy balance of biodiesel production is unsatisfactory.
Some references, for example, DE-A 102006024901, have disclosed the preparation of acrylic acid from propane, which is a raw constituent of natural gas. A disadvantage of such a method, however, is that propane is generally high unreactive.
The aldol condensation reaction of formaldehyde and acetic acid and/or carboxylic acid esters has been disclosed in literature. This reaction forms acrylic acid and is often conducted over a catalyst. For example, condensation catalysts consisting of mixed oxides of vanadium and phosphorus were investigated and described in M. Ai, J. Catal., 107, 201 (1987); M. Ai, J. Catal., 124, 293 (1990); M. Ai, Appl. Catal., 36, 221 (1988); and M. Ai, Shokubai, 29, 522 (1987).
U.S. Patent Publication No. 2012/0071688 discloses a process for preparing acrylic acid from methanol and acetic acid in which the methanol is partially oxidized to formaldehyde in a heterogeneously catalyzed gas phase reaction. The product gas mixture thus obtained and an acetic acid source are used to obtain a reaction gas input mixture that comprises acetic acid and formaldehyde. The acetic acid is used in excess over the formaldehyde. The formaldehyde present in reaction gas input mixture is aldol-condensed with the acetic acid via heterogeneous catalysis to form acrylic acid. Unconverted acetic acid still present alongside the acrylic acid in the product gas mixture is removed therefrom and is recycled to the reaction gas input mixture. Although US Patent Publication No. 2012/0071688 may disclose many details related to methanol oxidation and aldol condensation reactions, the reference discloses very little about the separation schemes necessary to effectively separate the crude condensation product mixtures.
Although the methanol oxidation reaction and the aldol condensation reaction are disclosed, there has been little if any disclosure relating to separation schemes that may be employed to effectively provide purified acrylic acid from the aldol condensation crude product, which contains significant amounts of formaldehyde, which is known to cause problems in the purification of acrylate products.
Thus, the need exists for a process for producing purified acrylate product, e.g., acrylic acid, which provides for efficient separation of purified acrylate product from the crude aldol condensation product.
The references mentioned above are hereby incorporated by reference.